1. Field of the Invention
The present invention relates to an optical communications system for transmitting and receiving an optical signal bidirectionally through a single common fiber-optic cable. More particularly, the present invention relates to a method for measuring the far-end reflectance of a fiber-optic cable used in a digital communications system capable of high-speed transport, such as IEEE1394 and USB2.
2. Description of the Related Art
A conventional optical communications technique using a fiber-optic cable(s) will be described.
FIGS. 6A and 6B are schematic diagrams for explaining a one-directional communications method using a fiber-optic cable.
In the one-directional communications method shown in FIG. 6A, information is transported as an optical signal from a first transceiver 1 to a second transceiver 2 via a fiber-optic cable 3. A transmitter 1a of the first transceiver 1 and a receiver 2b of the second transceiver 2 are connected to the fiber-optic cable 3.
Conversely, when information is transported as an optical signal from the second transceiver 2 to the first transceiver 1 via a fiber-optic cable 3, as shown in FIG. 6B, a transmitter 2a of the second transceiver 2 and a receiver 1b of the first transceiver 1 are connected to the fiber-optic cable 3.
Note that when an optical signal is transported only from the first transceiver 1 to the second transceiver 2, the receiver 1b of the first transceiver 1 and the transmitter 2a of the second transceiver become unnecessary. Conversely, when an optical signal is transported only from the second transceiver 2 to the first transceiver 1, the receiver 2b of the second transceiver 2 and the transmitter 1a of the first transceiver 1 become unnecessary.
To achieve both optical signal transport from the first transceiver 1 to the second transceiver 2, and optical signal transport from the second transceiver 2 to the first transceiver 1, as shown in FIG. 6C, the transmitter 1a of the first transceiver 1 and the receiver 2b of the second transceiver 2 are connected to each other via a single fiber-optic cable 3 while the transmitter 2a of the second transceiver 2 and the receiver 1b of the first transceiver 1 are connected to each other via another single fiber-optic cable 4.
Therefore, in the conventional one-directional optical communications method, two fiber-optic cables are required for full-duplex communications in which an optical signal can be transmitted and received bidirectionally between a pair of transceivers.
Hereinafter, a full-duplex communications method capable of transmitting and receiving an optical signal through a single fiber-optic cable will be described.
FIG. 7A is a schematic diagram for explaining a method for transmitting and receiving an optical signal bidirectionally through a single fiber-optic cable.
In this conventional bidirectional communications method, a single fiber-optic cable 13 is connected to a first transceiver 11 and a second transceiver 12.
The transceivers 11 and 12 comprise connectors 11a and 12a, respectively. A plug (not shown) is provided at each of end faces 13a and 13b of the fiber-optic cable 13, and is connected to each of the connectors 11a and 12a of the respective transceivers 11 and 12.
FIG. 7B is a schematic diagram showing the connectors 11a and 12a of the respective transceivers 11 and 12 and the end faces 13a and 13b of the fiber-optic cable 13.
An optical signal is transmitted from the first transceiver 11 to the second transceiver 12 in the following manner. The optical signal is applied from the transmitter 11b of the transceiver 11 via the connector 11a to the end face 13a of the fiber-optic cable 13. This optical signal is introduced into the fiber-optic cable 13 and transmitted to the second transceiver 12. The optical signal is applied from the end face 13b of the fiber-optic cable 13 connected to the connector 12a of the second transceiver 12 to the receiver 12c of the second transceiver 12.
Similarly, when an optical signal is transmitted from the second transceiver 12 to the first transceiver 11, the optical signal transmitted via the fiber-optic cable 13 from the transmitter 12b of the second transceiver 12 is applied to the receiver 11c of the first transceiver 11.
In this case, for example, an optical signal emitted by the transmitter 11b of the first transceiver 11 is transmitted through the fiber-optic cable 13 to reach the receiver 12c of the second transceiver 12. In this case, however, part of the optical signal is reflected by the end faces 13a and 13b of the fiber-optic cable 13.
FIGS. 8A and 8B are schematic diagrams for explaining the reflection of an optical signal by the end faces 13a and 13b of the fiber-optic cable 13.
As shown in FIG. 8A, the end faces 13a and 13b of the fiber-optic cable 13 are connected to the connectors 11a and 12a of the first and second transceivers 11 and 12, respectively. When an optical signal is transmitted from the first transceiver 11 to the second transceiver 12, as indicated by arrows C shown in FIG. 8B, a part of an optical signal incidnet to the fiber-optic cable 13 is reflected by the end face 13a (near-end reflection), and as indicated by arrows D shown in FIG. 8B, a part of an optical signal outgoing from the fiber-optic cable 13 is reflected by the end face 13b (far-end reflection). The optical signals reflected by the near-end face 13a and the far-end face 13b of the fiber-optic cable 13 are transported along with the original optical signal which is transmitted from the transmitter 12b of the second transceiver 12 to the receiver 11c of the first transceiver 11. In this case, the reflected optical signal presents noise on the optical signal.
Therefore, it is important to measure how much an optical signal is reduced by the near- and far-end reflections.
A far-end reflectance representing a reduction in an optical signal due to far-end reflection is calculated, for example, in the following manner. An optical signal is emitted from the end face 13b of the fiber-optic cable 13 into air, allowing far-end reflection. The amount of light of the optical signal received by the receiver 12c is measured. On the other hand, the end face 13b of the fiber-optic cable 13 is immersed in a liquid matching oil having the same refractive index as that of the core of the fiber-optic cable 13 so that far-end reflection does not occur at the end face 13b of the fiber-optic cable 13. In this situation, the amount of light of an optical signal received by the receiver 12c is measured. The far-end reflectance is calculated based on the two measured amounts of light of optical signals.
A plug or the like is attached to an end of the fiber-optic cable 13, which is connected to a connector. When the end face 13b of the fiber-optic cable 13 is immersed in a liquid matching oil, the matching oil is likely to penetrate between the plug and a core of the fiber-optic cable 13. Therefore, when a number of fiber-optic cables 13 are measured for far-end reflectance, the matching oil has to be removed from the end face 13b of each fiber-optic cable 13, whereby the working efficiency is reduced.
According to an aspect of the present invention, a method for measuring a far-end reflectance of a fiber-optic cable, comprises the steps of connecting an end face of the fiber-optic cable to a transceiver comprising a transmitter for transmitting an optical signal and a receiver for receiving an optical signal, transmitting an optical signal from the transmitter of the transceiver and receiving the optical signal reflected by the other end face of the fiber-optic cable, and measuring a first amount of light of the reflected optical signal, where the other end face of the fiber-optic cable is open to air, transmitting an optical signal from the transmitter of the transceiver and receiving the optical signal reflected by the other end face of the fiber-optic cable, and measuring a second amount of light of the reflected optical signal, where the other end face of the fiber-optic cable is made to contact a solid having the same or substantially the same refractive index as that of the fiber-optic cable, and measuring the far-end reflectance of the fiber-optic cable based on the first and second amounts of light.
In one embodiment of this invention, when the solid has the same refractive index as that of the fiber-optic cable, the far-end reflectance of the fiber-optic cable is calculated as:                               Far          ⁢                      -                    ⁢          end          ⁢                      xe2x80x83                    ⁢          reflectance                =                                            P1              -              P2                                                      A                xc3x97                B                            +              P1              -              P2                                xc3x97                      100            ⁢                          xe2x80x83                        [            %            ]                                              (        1        )            
where:
P1 is the first amount of light;
P2 is the second amount of light;
A is an output of light at the far-end face of the fiber-optic cable; and
B is a ratio of light received by the receiver.
In one embodiment of this invention, the solid is in the form of gel or an elastomer.
In one embodiment of this invention, the solid is contained in a container.
In one embodiment of this invention, the container comprises a lid having an opening. A tip portion including the other end face of the fiber-optic cable is inserted into the opening.
In one embodiment of this invention, a fixing member is provided in the opening provided in the lid, for fixing the tip portion of the fiber-optic cable to the opening.
In one embodiment of this invention, an inner side of the container is covered with an optical absorption material.
In one embodiment of this invention, when the solid has substantially the same refractive index as that of the fiber-optic cable, the far-end reflectance of the fiber-optic cable is calculated as:                               Far          ⁢                      -                    ⁢          end          ⁢                      xe2x80x83                    ⁢          reflectance                =                                            P1              -                              (                                  P2                  -                                      A                    xc3x97                    a                                                  )                                                                    A                xc3x97                B                            +              P1              -                              (                                  P2                  -                                      A                    xc3x97                    a                                                  )                                              xc3x97                      100            ⁢                          xe2x80x83                        [            %            ]                                              (        2        )            
where:
P1 is the first amount of light;
P2 is the second amount of light;
A is an output of light at the far-end face of the fiber-optic cable;
B is a ratio of light received by the receiver; and
a is a surface reflectance of the solid with respect to the fiber-optic cable.
In one embodiment of this invention, the solid is in the form of a plate, and the other end face of the fiber-optic cable is made to contact the solid in a slanting direction with respect to the solid.
In one embodiment of this invention, the solid is covered with a material having light blocking and light absorbing capabilities.
Thus, the invention described herein makes possible the advantages of providing a method for easily measuring far-end reflection of a fiber-optic cable, in which measurement can be easily repeated without reducing characteristics of the fiber-optic cable, resulting in the excellent working efficiency.